The present invention relates to a distance measuring device, and more particularly, to a digitized caliper device which eliminates the need for an incremented linear object to measure or mark distances.
U.S. Pat. No. 2,886,892 to Banfill, discloses a related device. A curved resistive element is attached to the pivotable arms of a caliper-type device so that the resistance of the portion of the resistive element between the arms is related to the angular spread thereof. A signal related to the resistance is directed to an external processing unit which displays the distance between the points of the caliper arms.
However, an inherent inaccuracy exists with the pivotable arm-type device described above. Assuming the resistance per unit length of the resistive element is constant, the resistance measured is proportional to the angular spread of the caliper arms. However, the distance between the tips of the arms is proportional to the sine of one-half the angle between the arms. Thus, the distance between the tips of the arms is not proportional to the angular spread of the arms or the resistance measurement related thereto. For example, when the angle between the caliper arms increases by a factor of 6 from 15 degrees to 90 degrees, the distance between the tips increases by a factor of 5.42. Since an increase by a factor of 6 in the angle between the arms does not produce an equivalent factor of increase in the distance between the tips, an inherent error results.
Thus, either this inherent error must be tolerated, or a resistive element whose resistance per unit length varies with the sine of one-half the angle between the caliper arms must be utilized. Although sinusoidal potentiometers are known in the art, they are expensive. The expense in this particular instance would be particularly significant since the potentiometers would have to be custom made for the calipers.
Furthermore, Banfill teaches the use of an external housing for the display, calibration and scaling circuitry--a distinct disadvantage in such a device. When attempting to scribe a circle or simply transfer a measurement, the cord connecting the mechanical component to the electrical component would certainly interfere with the ease of operation of the device. Furthermore, the cord, and particularly its connection to the mechanical and electrical portions, may be subjected to damaging stress.
In addition, Banfill teaches an analog display. This increases the amount of time necessary to read the device, and also makes the reading process less accurate. As a result of the increased concentration and time necessary to read an analog scale, the user of the device is more likely to experience eye strain and fatigue.
The present invention overcomes the problems inherent in the prior art. In the present invention, two reference devices are attached to a common beam, one being ridigly attached and the other being slidably attached. The beam also includes a resistive strip along one of its faces, which strip is electrically contacted by pickups attached to the slidable and fixed reference devices. The resistance between the pickups is directly proportional to the distance therebetween. The resistance is detected and displayed in digital form using circuitry which may be attached to the distance measuring device. In fact, in a preferred embodiment, the beam is glass and at least the digital display is embedded within the glass. Means are also provided for displaying the distance between the reference devices in any one of a plurality of units or in scaled form.
Thus, since the movement of the variable reference device with respect to the fixed reference device is linear, as is the resistane per unit distance of the strip, the inherent inaccuracy of a caliper device with pivotable arms is eliminated. Since the device need not be connected to an external unit, utilization of the device requires a minimum effort. The digital display further reduces the fatigue inherent in using such a device for extended periods of time.